Inspirations: Are We Facing the Great Filter?

  • The Fermi Paradox asks why we appear to be the only technological species in a galaxy of hundreds of billions of stars.
  • Robin Hanson proposed a ‘great filter’ that prevents life from developing further than we have.
  • If it involves the formation of complex life, humanity has already passed through it.
  • It is possible that we have yet to encounter it.

The cartoon that inspired the Fermi Paradox

It started with a cartoon. Specifically, a New Yorker cartoon of little green men making off with trashcans in their flying saucers. Most people who saw it would have chuckled at the suggestion of where all the missing trashcans go. One group of physicists having lunch at the Los Alamos Laboratories took it further, talking about how it explained why earth has never been contacted by aliens: it would ruin their clandestine trashcan theft.

Even among physicists, this was a distinguished group. It was only a few years since Edward Teller, Herbert York, Emil Konopinski and Enrico Fermi had worked together on the Manhattan Project. Among most people, it would be a little strange if the conversation turned to whether those flying saucers could travel faster than light. For these men, it would be remarkable if it didn’t.

In a pause between the numbers being slung around, Fermi looked up and asked, “But where is everybody?”

No one had to ask what he meant. Their minds worked along similar enough lines that they immediately understood that Fermi was wondering why, in a galaxy full of stars, no one had come to visit.

That’s one of several versions of how the question we know as the Fermi paradox was


Frank Drake in 2012 (Raphael Perrino Follow [CC / Flickr])


The Drake Equation

A decade later, in 1961, Frank Drake and his colleagues were discussing whether radio astronomy might detect an alien civilisation. Earth, after all, was leaking so much television and radio into space that it was effectively a giant beacon signalling the existence of intelligent life. Perhaps our distant neighbours liked their equivalent of Cary

Grant as much as Drake’s next door neighbours enjoy ours, in which case we might be able to detect them. Drake listed the factors affecting how many detectable civilisations there may be in our galaxy. The ‘Drake Equation’ argues that the number is a function of the following factors:

  • The rate of star formation in our galaxy.
  • How many of those stars have planets.
  • How many of those planets can support life.
  • How often life arises on planets that can support it.
  • How many of those planets develop technological civilisations.
  • How many of those civilisations develop a technology that is detectable from earth.
  • How long those civilisations continue to release detectable signals into space.

We can put a number on some of those variables, as Drake himself explained in 2003. We know, for example, that between four and nineteen stars are formed in our galaxy every year, and it’s estimated that around half of those produce planets. That’s a lot more planets than was expected when Drake formulated the equation

On the other hand, the length of time a civilisation releases detectable signals into space is probably far shorter than Drake originally believed. In 1961, it looked likely that we’d go on blasting radio waves into space for centuries. Since then, cable television and satellite


Our galaxy as we see it from our planet (inefekt69 [CC / Flickr])

relays has led to earth making a lot less noise now than it did then.

It’s full of stars

There are between a hundred billion and four hundred billion stars in our galaxy. In other words, the lowest estimate is one followed by eleven zeroes. The galaxy has existed for around fourteen billion years, which is about three times the age of the earth. We’re johnny-come-lately around here.

There has been plenty of time for life to get civilised and develop technology far more advanced than our own. Even if they’ve given up blasting their neighbourhood with radio waves, they could well have built the technology they’d need to come and visit. Voyager One has already left the solar system and several more probes are on their way. If we can do that with 1970s technology, what could we do with another couple of centuries behind us? And if we could, why haven’t the older kids on the block done it already?

Travelling at a mere tenth of the speed of light, it would take a civilisation around twenty million years to cross the galaxy. The galaxy has lasted seven hundred times that long so even if they’d stopped off for some sightseeing, anyone doing the rounds would have got round to us by now.

The Great Filter

In 1998, Robin Hanson approached the question of why no one has dropped by in much the same way as Drake approached the question of why we haven’t heard from anyone:

Consider our best-guess evolutionary path to an explosion which leads to visible colonization of most of the visible universe:

The right star system (including organics)

Reproductive something (e.g. RNA)

Simple (prokaryotic) single-cell life

Complex (archaeatic & eukaryotic) single-cell life

Sexual reproduction

Multi-cell life

Tool-using animals with big brains

Where we are now

Colonization explosion

Yet among hundreds of billions of stars, not one has spawned a species that has reached Hanson’s final stage. They’d be in the solar system with us if they had. Hanson concluded that there must be something interfering with that process. Something that prevents life moving from one stage to another. He called it the ‘great filter’, as it filters out most if not all candidates for that final stage.

Which begs the question of whether we are the product of life that has already passed through that filter or whether we will run into it before we get to the final stage.

The problem is that while we may infer that the great filter exists from our own existence and the lack of anyone else that we’ve heard from, we have no idea what it is. Or indeed, if it is only one filter or if there are many filters between a star’s formation and a species escaping its orbit, as Hanson himself explained in a TEDx talk:

The first steps

A recent paper by Aditya Chopra and Charles Lineweaver of the Australian National University described the filters that may appear at different stages of the journey from the first life to the next planet. There are plenty of stars in our galaxy and plenty of them have planets. Life on earth is based on hydrogen, oxygen, carbon, nitrogen, sulphur and phosphorus, which are among the most abundant elements in the universe. Even if our chemistry is the only possible basis for life, there’s plenty of it about on plenty of planets. If life could arise from different chemistries, there are even more candidates.

The first filter, or bottleneck as Chopra and Lineweaver call it, occurs with the question that has vexed biologists for decades: how do simple molecules become the complex chemistries of life. It’s not something that has ever been done under laboratory conditions, but no laboratory has ever been able to sustain those conditions for the billions of years the early earth did.


Our planet as it was when life first arose (NASA Goddard Space Flight Center [CC / Flickr])

We have no idea of what those first contraptions to qualify as life might have looked like. The simplest microbes around now would regard those first replicating blobs as a conveniently packaged source of nutrients so even if they reappeared somewhere, they wouldn’t last long. For all we know, they’re popping up all over the place and being digested by bacteria before we have a chance to notice them.

The Gaian bottleneck

The thrust of Chopra and Lineweaver’s argument revolves around what they call the ‘Gaian Bottleneck’, referring to Lovelock’s seminal work on the interaction between organisms and their environment, the much misunderstood Gaia hypothesis. Lovelock proposed that life on earth has been to stabilise the climate, which in turn has allowed life to continue.

For example, if more sunlight hits the earth, it stimulates more photosynthesis by plant-like phytoplankton in the oceans. A by-product of that photosynthesis is a chemical called dimethyl sulphoxide. As it hits the atmosphere, dimethyl sulphoxide reacts with oxygen, which is also a product of photosynthesis, and becomes sulphur dioxide. In the upper atmosphere, sulphur dioxide forms so-called ‘condensation nuclei’ around which clouds form. Clouds reflect sunlight so effectively, an increase in sunlight stimulates the process


A phytoplankton bloom of the Pacific Coast of North America, staining the sea green and regulating our atmosphere (eutrophication&hypoxia [CC / Flickr])

by which the sunlight is reflected. As the sun has increased its output by 20-30% since the first life appeared on earth, it would have scoured life from the land without something to limit how much of that output hit the earth’s surface.

We should be clear that contrary to some interpretations, Lovelock’s Gaia hypothesis does not place nature in a harmonious balance or that the earth is automatically regulated for the benefit of humanity. The vast majority of species that have ever lived could bear witness to that if they weren’t extinct.

A tale of three planets

Chopra and Lineweaver suggest that the great filter may in fact be the appearance of such mechanisms to regulate the climate, or rather the fact that they rarely occur. If life fails to evolve in a way that regulates the climate, life will become extinct as it would have done on earth if the earth’s surface had heated up as much as the sun has in the last billion years.

Earth, Mars and Venus all started with atmospheres of over 95% carbon dioxide, and they were all formed out of asteroids containing a mix of rock and water. All three of them are hit by enough sunlight that they could retain liquid water if their atmospheres were of the right mix of gases. Yet while earth retained so much of its water that it appears blue from


From left to right, Mercury, Venus, Earth and Mars. Spot the odd one out. (Kabsik Park [CC / Flickr])

any distance, all of Venus’s water has boiled off and the little left on Mars is frozen solid.

Chopra and Lineweaver suggest that the critical difference is that a Gaian feedback mechanism arose on earth. If life arose at all on Venus or Mars, it did not develop such a mechanism and the unstable climate subsequently wiped it out.

It is very unlikely that any life, however simple now exists on Mars or Venus. If life on earth is anything to go by – which, admittedly, is an untestable assumption – life doesn’t stay confined for long. It evolves into whatever space is available to it and it lives by the gases it draws in and expels, which leaves a signature in the atmosphere. The oxygen we breathe wouldn’t be in the atmosphere for long if plants stopped producing it. So if there ever was life on Mars, it is extinct.

Is there a memento mori on Mars?

As we send more and more robots to crawl over the surface of Mars, there is always the possibility that they will find evidence of the life of aeons past. They may find the petrified remains of some large animal or plant. They may even find discarded tools.

Nick Bostrum of Oxford University hopes they won’t:

The more complex the life we found, the more depressing the news of its existence would be.   Scientifically interesting, certainly, but a bad omen for the future of the human race.

At first glance, it seems an odd statement. ‘Scientifically interesting’ is a massive understatement. The discovery of alien life would be the greatest discovery in human


If life on Mars ever looked like this, we’re in trouble (QThomas Bower [CC / Flickr])

history, even if it’s extinct by the time we get to it. So why would it be a bad omen for us?

Because if life on Mars arose and became extinct, it’s probably because it hit the great filter. Anything that the current generation of Mars robots are able to find must at least have reached the point of multi-cellular life. Any sort of tool would imply that Martians reached much the same stage that we have. The nature of the tool doesn’t matter much. The few thousand years it took us to go from flint knapping to nuclear fission amount to an eyeblink in geological time, and we ourselves are evidence that a species that could produce simple tools like handaxes could go on to produce very complex ones like nuclear power stations, or indeed Mars rovers.

As there is no life currently on Mars, it has probably hit the great filter. If it never arose or was wiped out before it hit the stage we’re at now, the implication would be that we have already passed through the filter that has left Mars a lifeless red rock. If life as complex as us did arise and became extinct without getting off the planet, it would imply that we have yet to encounter the great filter but it is getting closer by the day.

Vexing as they are, the questions of how life arose and how Gaian feedback came to be do not refer to threats to our continued existence because the fact is that life has arisen and Gaian feedback has come to be. Global warming, incidentally, may perturb the Gaian feedback but it will take more than a few hundred parts per million carbon dioxide in the atmosphere to derail it completely. That’s not to say that Gaia will protect us from our own mistakes. We have a very difficult couple of centuries ahead of us. It just means that if we don’t make it, the earth will happily carry on without us.


Artist’s impression of the Milky Way Galaxy. We’re in there somewhere (Nick Risinger [Wikimedia Commons])

If we have passed through the great filter, we’ve joined a club so exclusive that we may be the only species in it. Better than that, we’ve already worked our way to the penultimate point of Hanson’s progression and if we ever work out how to get off our planet, we’re not likely to be treading on someone else’s lawn.

But what if life often gets to where we are now? For all we know, the galaxy could be teeming with living planets. We may be one among a huge number of planets, species and maybe even civilisations stuck on this side of the great filter.

King Coal

Another possible location for the filter lies in our recent past, in the development of the industry that made radio broadcasts and space travel possible. Our ancestors built our technology on the basis of the fossil fuels that are so abundant on this planet. Without easily accessible reserves of coal to draw energy from, we’d never have got out of the iron age. Those reserves came about through a quirk of evolution: Plants evolved lignin, the basis of wood, around 360 million years ago. At the time, there was a large tropical zone and continental drift put much of the earth’s land area inside it. The warm, wet conditions favoured both rapid plant growth and formation of coal from dead wood.

The coal party ended some 290 million years ago, with the appearance of white rot fungi that, unlike anything that had evolved before, could digest lignin. From that point onward, felled trees would decompose before they could form coal.

What that means to us is that our planet spent seventy million years storing fossil fuels for us to build a civilisation with. Every technology more complex than a blacksmith’s


A Carboniferous swamp: the basis of the industrial revolution (the paleobear [CC / Flickr])

forge is based on coal. Before long, we’ll have to break our dependence on it and turn to wind, solar, tidal or nuclear power but if that’s an option, it’s because we used coal to bootstrap our technology to this point.

It’s possible that the conditions that gave us that coal are rare enough to count as a filter in themselves. Perhaps lignin rarely arises or if it does, microbes that can digest it in a mere couple of million years. In either case, there wouldn’t be enough coal to even consider the steam engines and industrial foundries that allow us to look up and think we may get out there one day.

If our civilisation falls and our technology is lost, neither humans who survive it nor any intelligent species that evolves later will be able to build technology as we have. We’ve been inconsiderate enough to burn most of the easily available coal, and white rot fungi will ensure there won’t be any more.

The great what if?

Which brings us to the great problem in thinking about why we seem so alone. We know of one planet on which life has evolved, and we don’t know whether the processes that got us here are so rare as to be almost miraculous or so commonplace as to be normal. We can only extrapolate from what we know but we know so little that we don’t know if we face a great existential crisis.

It does appear likely that if we have a few more centuries of technological development at the rate of the last few, we will make our presence felt outside our solar system unless we


Voyager 1 leaves the solar system. Will it be the first of many? (NASA Solar System Exploration [CC / Flickr])

hit the great filter. We may never colonise a planet orbiting an alien star, but we probably will at least send out a few robotic probes to see if anyone’s out there. If we can even conceive of doing that, it’s strange that no one has done it in an ancient galaxy containing hundreds of billions of stars.

So have we passed through the great filter, or is there something looming in our future that will keep us confined to earth? Will we go on asking Fermi’s question of where everyone is until the filter extinguishes us, or will we head for the stars to look for anyone else asking the same question?

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Posted in Inspirations, Wednesday Pontification
2 comments on “Inspirations: Are We Facing the Great Filter?
  1. What a wonderful post! Interesting, informative, and fun to read. And I’m so glad you chose to include my “Martian Valentine”. He couldn’t have found a better home.
    – QThomasBower

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